What is the type of problem requiring to rate images on a scale?
The main distinction between tasks is 'classification' vs 'regression'. In classification you would try to identify the presence of a cloud or not in an image, if you want to predict the level of 'cloudness' with continuous values you are then performing a regression task. 

I'm not aware about state-of-the models specific for images, but you can potentially use whatever architecture you desire to perform regression, CNN, RNN, the only thing to pay attention to is the loss function you will use. There are specific functions for classifications (which usually use the argmax function to turn probabilities into labels) and for regression (the most used is the Mean Squared Error, the model try to approximate the continuous values directly).

For a quick overview of loss functions I suggest this easy tutorial 5 Regression Loss Functions. Hope it might be of use.